Re: [Qemu-devel] [PATCH 07/20] w64: Fix definition of setjmp

[Stefan Weil]

Am 15.04.2012 20:02, schrieb Blue Swirl:
> On Sun, Apr 15, 2012 at 17:47, Stefan Weil <address@hidden> wrote:
> > Am 15.04.2012 19:18, schrieb Blue Swirl:
> >
> > > On Sun, Apr 15, 2012 at 17:09, Stefan Weil <address@hidden> wrote:
> > > > Am 15.04.2012 19:02, schrieb Blue Swirl:
> > > >
> > > > > On Sun, Apr 15, 2012 at 14:13, Stefan Weil <address@hidden> wrote:
> > > > > > The default definition of setjmp which is implemented in MinGW-w64
> > > > > > cannot be used with programs like QEMU which call longjmp from
> > > > > > code without structured exception handling (SEH).
> > > > >
> > > > >
> > > > >
> > > > > We're currently compiling QEMU with -no-seh, is that correct for
> > > > > Mingw64?
> > > >
> > > >
> > > >
> > > >
> > > > Yes, that's correct. This code in configure is used for w32 and for 
> > > > w64:
> > > >
> > > > # Use ASLR, no-SEH and DEP if available
> > > > if test "$mingw32" = "yes" ; then
> > > >   for flag in --dynamicbase --no-seh --nxcompat; do
> > > >       if $ld --help 2>/dev/null | grep ".$flag" >/dev/null 
> > > > 2>/dev/null ;
> > > > then
> > > >           LDFLAGS="-Wl,$flag $LDFLAGS"
> > > >       fi
> > > >   done
> > > > fi
> > > >
> > > > See resulting file:
> > > >
> > > > bin/debug/w64/config-host.mak:LDFLAGS=-Wl,--nxcompat -Wl,--no-seh
> > > > -Wl,--dynamicbase -Wl,--warn-common -m64 -g
> > >
> > >
> > > Yes, but I meant that since Mingw64 uses SEH, does -Wl,--no-seh
> > > conflict with Mingw64 SEH usage somehow? If yes, should we disable
> > > no-seh for Mingw64 and could we also use plain setjmp() then?
> >
> >
> > Using --no-seh conflicts with the default which was set by MS
> > to use SEH on w64.
> >
> > I don't know whether SEH makes any difference for C applications
> > like QEMU. Typically C code does not need stack unwinding,
> > therefore I don't think that SEH is really needed. I think that
> > MS wanted to improve the support of languages like C++ and .NET
> > which need SEH in their standard.
> >
> > SEH increases the size of the exe file. If we only remove
> > --no-seh for w64, QEMU's JIT code will still not support
> > SEH, so I expect that longjmp will still crash when trying
> > to unwind the stack.
>
> Thanks for the explanation. Would similar hack as was done with GDB
> for ELF help, so one day stack unwinding would work for generated
> code?

I had a discussion with Kai Tietz from MinGW-w64 who gave me
an overview what would be needed for SEH.

Each function needs SEH information in special sections which
are normally allocated and filled with data by the compiler and
linker. It's also possible to manipulate that data at run time.

> > When QEMU is compiled to use TCI, that's no problem, and
> > stack unwinding works although we still compile using --no-seh
> > (I had a w64 version with TCI long before it worked with tcg/i386).
>
> OK. BTW, have you tried to compile QEMU with MS compiler, would we
> need a lot of changes to support that?

QEMU has lots of dependencies on GNU gcc, therefore a compilation
with Visual Studio would need a lot of changes. Compiling QEMU with
gcc restricted to C99 gives an overview of those changes.

I remember that there was once a report on qemu-devel that someone
tried using VS with QEMU.

I never tried it myself and are not planning to try it.